Ch 3 Lecture 2 Bonding Considerations

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Ch 3 Lecture 2 Bonding Considerations

• VSEPR Theory Continued
• Lone Pair Repulsion
• Lone pairs are as important as atoms in VSEPR
• Irregular structures result when central atoms
  have lone pairs
• Lone pairs (lp) occupy more space on the central
  atom than bonding pairs (bp)
• a. bp electrons are attracted to a second nucleus
• lp electrons are localized on the central atom
• (lpbp) repulsion is greater than (bpbp)
  repulsion
• Ammonia HNH bond angle 106.6o instead of
  109.5o
• Water HOH bond angle 104.5o instead of 109.5o

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• Subtle Geometric Influences
• Size of the central atom
• Larger central atom leads to longer bonds and
  less bpbp repulsion
• Smaller bond angles result
• Example H2O, H2S, H2Se, H2Te
• Electronegativity of outer atoms
• More electronegative outer atom pulls bp towards
  it
• Reduces bpbp repulsion and makes smaller bond
  angles
• Example PF3, PCl3, PBr3, PI3
• Size of outer atoms
• Very small outer atoms (H) may reverse expected
  trends
• Example SH2, SF2, SCl2
• Exact predictions are rarely needed, qualitative
  predictions are often enough.
• Table 3-4 gives geometric data

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• Choose the structure that allows maximum lone
  pair separation
• Example ClF3
• SN 5, so structure is base on trigonal
  bipyramid
• Three possible structures
• lplp most important eliminates B as a
  possibility
• 90o interactions are very unfavorable
• Angles gt 90o give about the same interaction
• Structure A has 6 90o lpbp interactions
• Structure C has 4 900 lpbp interactions

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• 2. Always place lone pairs in equatorial
  positions in SN 5 and SN 7

linear
bent
trigonal pyramid
bent
T-shaped
linear
see-saw
square pyramid
square planar
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• Remember lone pairs count in SN, but are left
  out in structure name
• Examples SbF4, SF5, SeF3
• Exercise 3-2
• Multiple Bonds
• Double and Triple bonds have greater repulsion
  than
• single bonds due to the large p-orbitals involved
• Multiple bonds are placed to minimize
  interactions, similar to lp
• Multiple bonds are placed equatorially in SN5,
  SN7
• lp gt mp gt bp
• Examples HCP, IOF4-, SeOCl2
• Exercise 3-3

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• II. Electronegativity
• Definitions
• Electronegativity power of an atom in a molecule
  to attract electrons
• Polar Bond between pure covalent (sharing e-)
  and ionic (transfer of e-)
• Homonuclear Diatomics completely nonpolar HH
• Heteronuclear Diatomics polar HCl
• Paulings Electronegativity Scale
• Definitions
• D Bond Dissociation Energy (AA ? A A)
• D Ionic Resonance Energy (kJ)
• cA Electronegativity of A
• Equations

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• F set at c 4.0 and other atoms determined from
  it
• Example Find D of HCl using table 3-1 (cH
  2.20, cCl 3.16)
• Exercise 3-4 Find D of HO. (HH 432kJ/mol,
  OO 213 kJ/mol)

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• Large c differences give ionic bonds small c
  differences give polar covalent bonds
• Actual c values come from averaging several
  compounds of the atom to remove experimental
  error
• Actual c values in a given molecule may vary from
  the average
• Noble gases have high x values because of high
  effective nuclear charges
• Small size
• Large nucleus

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• Polar Bonds
• Any bond between atoms of different
  electronegativities is polar
• Electrons concentrate on one side of the bond
• One end of the molecule is () and one end is (-)
• Calculating Dipole Moments
• Dielectric Constant Capacitance of Full Cell /
  Capacitance of Vacuum
• The orientation of polar molecules cancels out
  capacitance and this difference gives the
  dielectric constant
• The temperature dependence of this value lets us
  calculate dipole moment (m)
• m Qr units Cm or D(debye)
• Q difference in charge
• r distance of separation
• 1 D 3.38 x 10-30 Cm
• Complex molecules vector addition of all m
• Not very accurate (table 3-5) because of
  variation lengths and angles
• Lone pairs have a large influence on dipole
  moment as well

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• Examples
• Large dipoles result from lone pair reinforcement
  of bond dipoles
• Small dipoles result from lone pair cancellation
  of bond dipoles
• Symmetric molecules have have no net dipole

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• Hydrogen Bonding
• Hydrogens bonded to an electronegative atom (O,
  N, F, Cl, Br,) can form a weak bond to the lone
  pair on another electronegative atom
• Water boils about 200 oC higher than expected
  without this interaction
• Ammonia does not H-Bond with itself well, instead
  forming a NN dimer

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• Unusual properties of water due to H-Bonding
• Freezing point gtgt similar compounds
• Decreases in density on freezing
• H-Bonding network gives an open structure
• Ice floats (think biology and geology)

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• Other H-Bonding Phenomena include Protein and
  Nucleic Acid Folding